1. Field of the Invention
The present invention relates to optical interferometric length measurement. More particularly, this invention pertains to a method and device in which a high coherence length beam or beam component stabilized with respect to a reference section is split and then recombined after the components traverse reference and measuring sections, the interference pattern is detected, the resulting signal amplified and the measuring signal generated in accordance with a predetermined modulation pattern.
2. Description of the Prior Art
Prior art optical interferometric length measuring systems employ a stabilized, coherent laser light source whose output is split into two components, a reference beam and a measuring beam. The reference beam is retroreflected after traversing a fixed section while the measuring beam is reflected at the measuring object. Two retroreflected partial beams are recombined and directed to an interference with the period xcex/2 where xcex is the wavelength of the laser light. By measuring the interference phase with an accuracy of 10xe2x88x924, it is possible to achieve resolution in the sub-nanometer range. However, the measuring signal obtained is ambiguous, as it repeats itself with a period of xcex/2. A disadvantage for length measurement is the necessity to start with an accurate zero mark (in regard to spatial position) whose periods must be counted and stored. This measuring principle is known as incremental length measurement.
FIG. 4 illustrates the principle of prior art three-beam interferometry in which three (partial) beams are employed to obtain a signal for laser stabilization from the reference section between a first reflector and a second reflector, and the measuring signal from a measuring reflector.
A prior art arrangement (see DE 43 14 486 C2) for three-beam interferometry for absolute length measurement is illustrated in FIG. 3. The illustrated absolute measurement interferometer arrangement 1 has two tunable lasers 2, 3. The laser 2 is modulated by means of an operating current supply (not shown) in a wavelength region of its characteristic free from mode jumps whereas the laser 3 is operated at a fixed wavelength. A measuring interferometer 4 is provided in which an interferometer arm 5 forms the actual measuring section. The generation of at least two mutually interfering component beams 6, 7 is accomplished by photodetectors 11 and 12. The. photodetectors 11 and 12 are connected to counting electronics (not shown) while the lasers 2 and 3 are connected to a laser wavelength control device (again not shown). A control interferometer 13 whose control section 14 is of constant length (shorter than half the length of the measuring section) is provided in the immediate vicinity of the measuring interferometer 4. Otherwise, the control interferometer 13 corresponds to the measuring interferometer 4.
The beam of the same laser 2, 3 is applied to the measuring interferometer 4 and to the control interferometer 13 by means of a primary beam splitter 15 and a reflector 16. Coupling of the beam of the laser 2 is performed via a reflector 17. The determination of the absolute distancexe2x80x94Labs can be performed, for example, by using the measured residual phases xcfx861 and xcfx862 in an interval about the distance determined by the method of continuous tunability to determine the distance for which the following equation:
Labs=(n1+xcfx861)(xcex1/2)=(n2+xcfx862)(xcex2/2)
is most effectively satisfied with the interval being greater than twice the expected measuring uncertainty and less than half the synthetic wavelength. In the case of continuous transition from one wavelength to the other, the interferences xcex94n traversed are counted:
xcex94n=n2xe2x88x92n1.
It follows from the above that:
n1=(xcex94n+xcfx862xe2x88x92xcfx861)(xcex1/(xcex1xe2x88x92xcex2))
Thus, given known and stable wavelengths xcex1 and xcex2, n1 can be determined by measuring xcex94n, xcfx861 and xcfx862. Consequently, the length L of the measuring section can be measured in absolute terms or calculated in accordance with the above formula.
According to this prior art method, it is necessary to measure the ordinal number difference xcex94n of the interference as well as the two residual phases xcfx861 and xcfx862. Absolute accuracy is therefore a function of the accuracy of the residual phase measurements, xcfx861 and xcfx862. In the prior art, residual phase measurement is accomplished by measuring the intensity of the interference signal for the controlled wavelengths xcex1 and xcex2. Absolute length measurement is thereby limited to an accuracy of xcex/100 (compare DE 43 14 486 C2, column 8). Moreover, measuring accuracy is a function of the constancy of the light intensity, which increases the outlay.
It is therefore an object of the invention to provide apparatus and a method for absolute interferometric length measurement with substantially enhanced resolution and accuracy.
Another object of this invention is to achieve the above object at as little additional expense as possible.
The present invention addresses the preceding and other objects by providing, in a first aspect, an improvement in a method for optical interferometric length measurement in which a beam, or beam component, of a laser of high coherence length that is stabilized with respect to a reference section is split into two partial beams. One of the partial beams is reunited after traversing a reference section while the other is reunited after traversing a measuring section. The interference pattern is detected by a detector and the resulting signal is amplified and a measuring signal is generated in accordance with a prescribed modulation pattern. The laser is sequentially stabilized in time with respect to at least two different wavelengths xcfx861 and xcfx862 for absolute measurement of the length L of a measuring section, the number xcex94n of the interferences traversing the detector counted during transition from a first to a second wavelength, and the absolute length L then being calculated as a function of phase measurements xcfx861, xcfx862 for the two stabilized wavelengths xcex1, xcex2.
The improvement comprises the step of performing the phase measurement in a resetting control loop that compensates the phases.
In a second aspect, the invention provides a device for absolute optical interferometric length measurement. Such device includes a four-beam interferometer in which a light beam emanating from a laser is split in a 2xc3x972 coupler into two partial beams onto a reference channel and a measuring channel separate therefrom.
The two partial beams are each split in an integrated optical chip into two further partial beams that traverse a phase modulator that is integrated in the chip.
A processor is provided for applying different frequencies for the reference and measuring channels. A closed control loop is provided for measuring the phases of the reference and measuring channels by compensation at the phase modulators. A control circuit is provided for controlling the laser wavelengths (xcex1, xcex2) and the output of such processor is applied to the control circuit.
The preceding and other features of this invention will become further apparent from the detailed description that follows. Such description is accompanied by a set of drawing figures. Numerals of the drawing figures, corresponding to those of the written description, point to the features of the invention with like numerals referring to like features throughout.